They're the foundation stones of the fallen Lattice of Heaven*.
**Completely uncredited speculation in a sidebar on p. 36 of The Plane Above.
For all the flak 4E tends to catch for the way it rewrote the cosmos, it mostly made big broad strokes and left the rest undefined for the GM to fill in as they suit. Like, it's not saying that astral diamonds are being deliberately placed in circulation so that when the Game of Making rediscovers the secret of the Lattice of Heaven in Hestavar, there'll already be a critical mass of raw material to reconstruct the thing, assembled and brought there voluntarily out of self-interest and ambition. But it's not not saying that, you get me?
Astral Diamonds: Origins
As a means of exchange, astral diamonds absolutely originate in 4E. 3E's solution to players needing giant piles of money in the Epic Level Handbook was essentially a system of promissory notes called favors, operating on the principle that hiring someone to cast a spell for you had predictable pricing. (Epic Level Handbook p.114, "Managing Wealth".)
This is not to say nobody ever put the words "astral" and "diamond" together before then. Like, look: chaos pearls, fey rubies, shadow sapphires. Doesn't that fire you up to go collect some of those? And I'm pretty sure I just made all of them up.
But as the highest form of currency, the 10,000 gold piece, they made their debut in 4E, on the PHB p.212, "Coins and Currency".
Astral Diamonds: Fun Facts
- Actual "fancy carbon"-style diamonds are prized for their colorlessness and clarity, like pure water. Are astral diamonds held to the same standard, or are they prized for their likeness to the "waters" of the Astral Sea?
- Astral diamonds weigh 10 to the gold piece, so one pound of astral diamonds contains 500 astral diamonds, or 5,000,000 gold pieces - more than enough to buy any magic item that moves on the open market, since level 30s top out at a little over 3 million.
- As treasure (4E DMG p.124), astral diamonds start showing up in loot parcels beginning at level 24, in tens and twenties. However, it's also noted that they are gems in their own right, and while there's no 10,000 gp gem slot, it's easy enough to imagine them taking the place of two 5,000 gp gems, or serving as the centerpiece gem or gems to a suitably valuable piece of art, putting singleton astral diamonds well within the bounds of possibility to encounter as early as level 14.
Astral Diamonds: Made of Residuum, Because Numbers? (No.)
You can find this sentiment all over blogs and message boards from the early days of 4E: astral diamonds must be made of residuum since the same weight is worth the same amount. The equivalence is simple, satisfying, and, sadly, wrong.
Residuum was also introduced in 4E (PHB 225), as the aftermath of the Disenchant Magic Item ritual and a universally useful ritual ingredient in its own right. One gold-piece weight of residuum is worth 10,000 gp, which sounds like a lot, and it absolutely is, but high-end rituals like the level 28 True Portal will run you 50,000 gp a shot.
A gold-piece weight of residuum is 10,000 gp, and an astral diamond is 10,000 gp, but a gold-piece weight of astral diamonds is 10 astral diamonds, or 100,000 gp. The equivalence is a lot less easy to draw when it's off by an order of magnitude.
If you wanted to you could still go for it, of course. Gemstones have value in their cut and clarity - a cut diamond is worth more than an equivalent mass of diamond dust. But that's just an "if you wanted to", and you can want a lot of things.
Astral Diamonds: Powered By A Forsaken Child? (Also no.)
That same sidebar on p.36 of The Plane Above notes they're mined primarily in Celestia and Hestavar, the two major good-aligned domains. Tearing up souls for monetary gain is absolutely something the other domains would more actively pursue if they were able. And while it is noted in The Plane Above that many outsiders, those dead failed by the Lattice of Heaven, will often immure themselves in the border islands as they fade away, that's just regular stone - those souls that can get into a celestial domain have another destination; for example, souls in Celestia head for Chronias and depart to an unknown destiny across the Bridge of al-Sihal.